# Understanding the n-step off-policy SARSA update

In Sutton & Barto's book (2nd ed) page 149, there is the equation 7.11

I am having a hard time understanding this equation.

I would have thought that we should be moving $$Q$$ towards $$G$$, where $$G$$ would be corrected by importance sampling, but only $$G$$, not $$G-Q$$, therefore I would have thought that the correct equation would be of the form

$$Q \leftarrow Q + \alpha (\rho G - Q)$$

and not

$$Q \leftarrow Q + \alpha \rho (G - Q)$$

I don't get why the entire update is weighted by $$\rho$$ and not only the sampled return $$G$$.

• Thank you @nbro for the edit, I was a bit lazy with the equations :) Apr 5 '19 at 14:30
• Hi Antoine! Please, next time try to at least put some more effort to write these equations! Whenever I can, I try to write them nicely, but I would prefer if every user does it for its own questions/answers, of course!
– nbro
Apr 5 '19 at 14:31
• Will do, thanks Apr 5 '19 at 14:35

Multiplying the entire update by $$\rho$$ has the desirable property that experience affects $$Q$$ less when the behavior policy is unrelated to the target policy. In the extreme, if the trajectory taken has zero probability under the target policy, then $$Q$$ isn't updated at all, which is good. Alternatively, if only $$G$$ is scaled by $$\rho$$, taking zero probability trajectories would artificially drive $$Q$$ to zero.